Photo recording media which are conventionally known as rewritable photo recording media capable of writing and erasing include a phase transition type, a photochromic type, and a magneto-optical type. Of these rewritable photo recording media, the magneto-optical type recording media are characterized by high recording speed and excellent resistance to repeated use.
Conventional magneto-optical recording media comprise a substrate having thereon a magneto-optical recording magnetic film (hereinafter simply referred to as a magnetic film) comprising a rare earth metal-transition metal film (hereinafter referred to as an RE-TM film) or an MnBi type film, and those using an RE-TM film are excellent in general characteristics. However, these films are inferior in corrosion resistance and liable to undergo deterioration arising from corrosion, such as change of coercive force, reduction in angle of Kerr rotation, and pitting, which leads to reduction in C/N (carrier to noise) ratio and increase in error ratio when used as a recording medium.
In order to solve this problem, it is known to provide a coating film on one or both sides of the magnetic film to improve corrosion resistance.
Where a coating film is formed on the light incident side of the magnetic film, it also brings about enhancement of Kerr effect, i.e., an increase of Kerr rotation angle, of the magnetic film, but the coating film must have a large refractive index before the Kerr effect enhancement can be obtained.
On the other hand, it is desired that the coating film formed on a magnetic film and/or a substrate made of a synthetic resin, glass, etc. should be chemically stable and capable of blocking oxygen or moisture, and exhibit good affinity for the substrate or magnetic film. When, in particular, in using a synthetic resin, e.g., polycarbonate and acrylic resins, as a substrate, the coating film is keenly demanded to be impermeable to oxygen, moisture, etc. present in the substrate or permeating through the substrate while exhibiting good affinity for the substrate and magnetic film. Where a protective coating film is provided on the light incident side of the magnetic film, it is also required to have transparency enough for incident light to sufficiently reach the magnetic film as well as a high refractive index for taking advantage of the above-described Kerr effect enhancement function.
In addition, it is also demanded for magneto-optical recording media to work at a higher speed in order to increase a speed of information transfer.
Conventionally reported coating films meeting either one or both of the purpose of magnetic film protection and the purpose of Kerr effect enhancement include films of oxides, e.g., SiO.sub.2 and Al.sub.2 O.sub.3, and films of nitrides, e.g., AlN and Si.sub.3 N.sub.4. It has been point out, however, that magneto-optical recording media having such a coating film on a magnetic film have the following disadvantages That is, coating films of an oxide, e.g., SiO.sub.2 or Al.sub.2 O.sub.3, release oxygen readily oxidizing the rare earth metal to cause deterioration of RE-TM film characteristics. On the other hand, nitrides, e.g., AlN and Si.sub.3 N.sub.4, contain no oxygen and, hence, have satisfactory protective performance without causing oxidation of the magnetic film. However, coating films of AlN lack stability against moisture and those using SiN exhibit poor adhesion to a substrate and are liable to undergo peeling off.
Thus, under the present situation, a coating film satisfying various demands stated above has not yet been developed. Moreover, recording media using the conventional coating films have insufficient sensitivity in working at a high speed.